This section provides background information related to the present disclosure which is not necessarily prior art.
Inertial drives are known from the prior art, for example from WO2008/087469 A2. Such inertial drives comprise, in particular, stick-slip drives, that is to say drives which are distinguished by a continuously repeating alternation between a stick phase, in which the element to be driven is driven along by the movement of the driving element, and a slip phase, in which the element to be driven and the driving element slide against one another, and the element to be driven is presently not driven along, or is driven along only negligibly, by the movement of the driving element.
In the case of piezoelectric stick-slip drives, a piezoelectric actuator element is charged with a periodic voltage, in particular a high-frequency sawtooth voltage. The high-frequency expansion and contraction of the actuator element effected by the voltage is transmitted, via a friction element arranged on said actuator element, to a friction body such that the friction body is moved during a deflection of the actuator element in a stick phase, in which static friction exists between the friction element and the friction body, whereas, in a slip phase, sliding friction exists between the friction element and the friction body, such that the friction body is not driven along, or is driven along only to a very minor extent, by the movement of the friction element.
During the stick phase, the acceleration or the movement speed of the actuator element is in this case configured such that, owing to the forces that act in the frictional contact between the friction element and friction body, there is no resulting sliding friction or only negligible resulting sliding friction, such that the friction body is in any case driven along by the friction element as a result. By contrast, in the slip phase, the acceleration or movement speed of the actuator element is so high that the forces in the frictional contact between friction element and friction body are no longer sufficient for the friction element to drive the friction body along, and, owing to the inertia of the friction body, a relative movement between the friction element and friction body (that is to say sliding) occurs. Owing to the repeating sequence of the above-depicted movement processes, which lead to only a very short movement travel of the friction body per stick-slip cycle, the overall result is a movement travel that is limited only by the length of the friction body.
There is often the boundary condition of an only very small and often limited structural space for the installation of the inertial drive depicted above into a corresponding superordinate system. Arbitrary miniaturization of the inertial drive is however not possible for numerous reasons. For example, it is necessary for the actuator element to be supported by way of one of its end sections on a relatively large mass, such that the kinetic energy of the actuator element owing to the expansion thereof is transmitted as completely as possible, and with the least possible losses, to the transmission device bearing against the opposite end section of the actuator element—generally a spring element or a spring device—such that only the transmission device is set in motion, and said motion is ultimately transmitted to the element to be driven or to the friction body. Since it is thus the case that the mass on which the actuator element is supported must not undershoot a certain value, it is also the case that the corresponding support section must not undershoot a certain size or extent.
On the other hand, the mass of the transmission device, as an accelerated mass, should be as small as possible, which would duly be advantageous for miniaturization and the transmission of movement but is implementable only to a limited extent with solutions known from the prior art. Here, the transmission unit must transmit the generally relatively high normal force in the friction pairing of friction element and friction body, transmit the actuator movement with the greatest possible rigidity, and compensate for unevennesses and fluctuations in orientation of the element to be driven.
Furthermore, the arrangements or devices known from the prior art for such inertial drives are normally of very filigree design, and are therefore normally manufactured by way of an erosive process. The transmission device can then be manufactured only from materials which have only moderate and altogether unsatisfactory spring characteristics.
The inertial drives discussed above furthermore often require a relatively high axial mechanical preload or compressive stress acting on the actuator element. In the case of miniaturized inertial drives, the application of such a force or preload is complicated and technically difficult to implement owing to the limited space conditions.
It is therefore the object to reduce or eliminate the abovementioned disadvantages of existing inertial drives. In particular, it is the object to provide an inertial drive which can be miniaturized to a significant extent and, here, functions with adequate robustness. At the same time, it is sought to make it possible for the inertial drive to be produced relatively easily and inexpensively. It is also sought to realize an inertial drive with an easily adjustable normal force with which the friction element is pressed against the element to be driven, at the same time with as high as possible an axial preload or compressive stress on the actuator element. It is sought to make this possible even in very constricted installation spaces, in conjunction with a simple construction composed of easily miniaturizable elements.
The above-stated objects are achieved by way of an inertial drive comprising at least expedient embodiments and refinements.
Accordingly, an inertial drive having a frame element and having a variable-length actuator element, preferably a piezoelectric actuator element, inserted into the frame element is taken as a starting point. Here, the frame element has a support section with a support surface, against which the actuator element bears by way of one of its two opposite end sections or by way of the corresponding end surface. Furthermore, the frame element has a deformation section with an abutment surface against which the actuator element bears by way of the other end section or by way of the corresponding end surface. The deformation section of the frame element furthermore has a joint section which is provided primarily for mounting the deformation section movably relative to the support section. A flat, elongate and preferably planar spring element is arranged or fastened by way of its ends on the deformation section of the frame element, wherein the spring element has a friction section on its opposite free end. Furthermore, the inertial drive has a friction body for being driven, which friction body is in direct or indirect mechanical or frictional contact with the friction section. The variation in length of the actuator element gives rise here to a rotational movement of the deformation section about the joint section, and said movement is transmitted via the spring element to the friction section for the purposes of driving the friction body for being driven, wherein the friction section is arranged spaced apart from the abutment surface in a direction pointing away from the abutment surface. In this context, and likewise in the context of the further description, “spaced apart” refers to a positive, non-zero spacing. In the present context, the expression “in a direction pointing away from the abutment surface” describes a direction which points away from the abutment surface in the direction of the support surface, such that the friction section is arranged either in the region in which the actuator element is also situated, and is then correspondingly situated opposite said actuator element, or else the friction section is arranged behind the actuator element or behind the support surface in the direction pointing away from the abutment surface in the direction of the support surface.
Owing to the specific arrangement or position of the friction section, which is not arranged spaced apart from the actuator element or spaced apart from the abutment surface in a direction pointing away from the support surface and pointing toward the abutment surface in the manner known from the prior art, but which is arranged either in the region of the actuator element, and thus so as to be situated opposite the latter, or behind the actuator element, and spaced apart therefrom, or behind the support surface, in the direction pointing away from the abutment surface and in the direction of the support surface, it is the case, for a given structural space for the inertial drive, that significantly greater design freedom is provided with regard to the frame element. In this way, the geometric extent and thus the mass of the support section can be selected to be relatively large despite the generally small installation dimensions, which benefits the optimized operation of the inertial drive. The force generated by the actuator element acts with the same magnitude on the support and deformation sections. If the actuator element changes in length and thus performs work, energy is transmitted into the support section and into the deformation section. Here, the amount of energy transmitted corresponds to the reciprocal mass ratio of the elements involved. The lighter the deformation section and spring element are in relation to the support section, the more energy is ultimately transmitted to the friction section of the spring element, and the less energy flows in an undesired manner into the support section.
Through the use of a flat, generally planar and elongate spring element, considerably simplified production of the inertial drive according to the invention in relation to the prior art is realized. Furthermore, the spring element and the specific arrangement thereof within the inertial drive permits targeted adjustment of the preload or compression force acting on the actuator element inserted in the frame element. This applies equally to the normal force acting on the friction section in the direction of the element to be driven. The targeted adjustment of said forces is essential for the reliable operation of the inertial drive.
It may be advantageous for the mass of the support section to amount to at least five times the mass of the deformation section. With such a mass ratio, the change in length of the actuator element or the corresponding force is converted in an effective manner, virtually without losses, into a movement of the deformation section, wherein only a negligible (opposite) movement of the support section is generated as a result. Here, the higher the mass of the support section in relation to the deformation section, the more effective the above-described deformation or conversion of movement.
It may likewise be advantageous for the joint section to be formed in one piece with the frame element and to form a flexure joint. In this way, the joint section can be realized in a very small size and with little usage of material. Aside from the desired small dimensions, the mass to be moved in the deformation section is thereby kept small. For piezoelectric actuators with typically short strokes and high forces, flexure joints are particularly advantageous because they operate without friction and transmit very high forces with minimal hysteresis.
It may also be advantageous for the frame element and the spring element to be arranged relative to one another such that the spring element has a permanent preload directed toward the surface of the friction body, and said preload is transmitted via the frame element to the actuator element and effects a permanent pressure load on the actuator element, which pressure load is adjustable by way of the deformation of the spring element. It is thus possible in a relatively simple manner for both the preload directed toward the surface of the friction body and the pressure load acting on the actuator element to be adjusted. The adjustment of the preload directed toward the surface of the friction body, or the corresponding normal force, is in this case important for the frictional contact between the friction section of the spring element and the friction body. The compression force acting on the actuator element must be dimensioned such that any tensile force acting on the actuator element is always lower than the applied compression force.
It may furthermore be advantageous if the frame element has a counterbearing section and a bearing section, wherein the spring element lies on the bearing section, and the section of the spring element between the counterbearing section and the bearing section forms a clamping section with a non-zero clamping length Ls.
It may furthermore be advantageous if the spring element forms, between the bearing section and the friction section, a spring section with a non-zero spring length LF, and the spring length is at least twice the clamping length.
It may furthermore be advantageous if the compression force acting on the actuator element is at least as high as the normal force acting on the friction body and/or on the friction section. The optimum actuator preload is in this case dependent on the actuator type, but is typically considerably higher than the material-dependent optimum normal force in the friction pairing of friction section and friction body. The actuator preload is correspondingly dimensioned in order to protect the actuator element against tensile stresses. In the case of the inertial drive according to the invention, in particular as loading of the inertial drive increases, i.e. as the speed of the variation in length of the actuator element increases, so does its protection against tensile stresses. Inadvertent destruction of the actuator element during the assembly process is made less likely.
Furthermore, it may be advantageous if the spring element is provided as a separate element, and the counterbearing section is designed such that the spring element can abut against it in a supporting manner. This facilitates the production of the inertial drive and permits design modifications in a simple and inexpensive manner. Under some circumstances, it is sufficient here for the spring element to merely bear against the counterbearing section without an engaging action or engaging-behind action or other fastening being realized, such that the corresponding surfaces are pressed against one another.
Here, the clamping length LS has a major influence on the normal forces generated in the bearing section and counterbearing section. Adequately high normal forces in the bearing section and in the counterbearing section secure the separate spring element against slippage, and permit the reliable transmission of the movement of the actuator element to the friction section.
Furthermore, the ratio of spring length to clamping length has a major influence on the resulting friction forces on the counterbearing section and on the bearing section, which friction forces, in the case of typical friction coefficients of the friction pairing of spring element and frame element, reliably prevent slippage of the separate spring element on the corresponding bearing points.
It may furthermore be advantageous if the spring element has a width which varies over its length, wherein the width within the clamping section increases from the receiving section to the bearing section, and the width narrows at least in sections proceeding from the bearing section in the direction of the friction section. The increasing width within the clamping section gives rise, in said region, to adequately firm clamping of the spring element in the frame element. An aim of the variation of the width of the spring element is the realization of as large as possible a region of homogeneous stress distribution within the spring element. The larger the region of homogeneous stress distribution that can be realized, the more flexible the spring element becomes, that is to say the greater the bend angle of the spring element that can be selected. In the case of a typical design of the inertial drive according to the invention, for a given normal force on the friction section of the spring element, the deformation of said spring element amounts to approximately 100 times the actuator stroke. As a result, the deflection of the actuator element during operation gives rise to a fluctuation of the normal force by only ±1%.
It may furthermore be advantageous if the width of the spring element has a minimum between the bearing section and the friction section. In order to realize as large as possible a region of homogeneous stress distribution, as already described above, the spring element becomes wider toward the receiving section. However, it is also the case that the stability of the spring element should increase in the direction of the friction section in order to follow the movements of the actuator element as directly as possible. The two demands change their priority at the minimum.
It may furthermore be advantageous if the width of the spring element is constant in the region of the bearing section. In this way, for a given normal force load, the mechanical stresses in the spring element increase around the bearing section. Under the action of the normal force, the spring element will curve more intensely locally in the bearing region. The elastic bend that is generated additionally prevents the slippage of the spring element on the bearing section.
It may moreover be advantageous if the friction section of the spring element has a friction element which is inserted into the spring element.
It may furthermore be advantageous if the ratio between the length LAAFK of the lever between the bearing section and the joint section and the length LASFK of the lever between the center of gravity of the actuator element and the joint section lies between 1.2 and 3. With such a lever ratio, the inertial drive can be better adapted to an existing actuator. Furthermore, for the same vibration frequency of the actuator element, with a reduction of the thrust force, the speed of the inertial drive or of the element to be driven can be increased.
Furthermore, it may be advantageous if the frame element is formed in one piece with a superordinate structure that supports the frame element. This may be advantageous in the context of an integral and/or miniaturized design of the inertial drive.